Projection and electronic display systems are widely used to display image content. In the case of projection systems, whether the traditional film based systems, or the newer electronic systems, light from a single light source (typically a lamp) is directed to an image modulation element (such as film or one or more spatial light modulators) that imparts image data to the transiting light. Typically, the film or light modulator arrays are then imaged to the display surface or screen.
For a variety of reasons, including light efficiency, expanded color gamut, increasing lifetime and reducing ongoing replacement costs, there has been continuing pressure to replace the traditional lamps (such xenon arc, tungsten halogen, fluorescent, or UHP) with solid state light sources (such as lasers or LEDs). However, to date, the great desire for laser based projection systems has been unfulfilled as compact, robust, low to modest cost, high power, visible wavelength laser technologies have not emerged in a commercializable form, particularly for green and blue. Very recently, this obstacle has started to disappear, as companies such as Laser Light Engines (Salem, N.H.) and NECSEL (Milpitas, Calif.) have demonstrated prototype or early product laser devices. For example, NECSEL offers green (535 nm) and blue (465 nm) Novalux laser arrays, each of which provides 3-5 Watts of optical output power. At these power levels, and allowing for system efficiency losses, a modest sized projector (1500 lumens output) for a large conference room or a home theatre, can be achieved using a single compact laser device per color. However, in the case of motion picture theaters, the on-screen luminance requires 10,000-40,000 lumens or 40-170 Watts of combined optical power, depending on screen size and screen gain. Presently, these power levels can only be accomplished by optically combining the output of multiple laser arrays in each color channel, although eventually compact laser technologies may advance to the point that 40-120 Watts of optical power is available from a single laser device in each color. Potentially each approach has its advantages and disadvantages, relative to trade-offs of simplicity, cost, and susceptibility to laser failure. At present, however, approaches using single laser devices per color are not commercially feasible.
In comparison to lamp sources, which are typically operated CW (continuous wave output), laser sources are more complex structurally and operationally. Laser sources often benefit relative to lifetime and efficiency, by being pulsed or temporally modulated. In some cases this can be advantageous for system efficiency, as laser light need not be produced and wasted during the dark times that typically occur between image frames or sub-frames. However, in many cases, the available lasers can be reliably operated within only a limited range of PWM parameters, relative to frequency and duty cycle. This is important, as a variety of issues can emerge when pulsed laser light (from one pulsed laser, or a multiplicity of laser sources) illuminates light modulating light valves (such as liquid-crystal-on-silicon (LCOS) devices or digital micro-mirror array devices (DMD) devices). As a first example, image artifacts can occur from the interaction of the laser pulse modulation from the pulsed lasers sources with the temporal modulation of the spatial light modulator (SLM) array pixels, such that image code values or grey scale levels may not be faithfully reproduced if the individual or aggregate laser pulsing provides insufficient light during the projection of image content for each given frame. Secondly, the individual or aggregate pulse characteristics of the laser sources can also change due to operational fluctuations or degradation, which then makes the laser to SLM temporal or pulse interactions unstable. Ultimately, such laser source problems can affect the display color balance or white point, and can also affect the color rendition accuracy. Therefore, it would be valuable to provide a laser projection display in which the laser pulse modulation can be adaptively modified in response to changes in the performance of individual lasers, while maintaining the aggregate pulsed laser performance and a low impact relative to image artifacts.
Pulsing of lasers and other light sources for image display is known in the imaging arts. For example, laser sources can be directly modulated in raster scanning image projection systems or line scanning printing systems to impart image data into the scanning light beam. Numerous direct laser modulation circuits have been developed employing a variable combination of pulse width modulation (PWM) and pulse amplitude modulation (PAM). For example, U.S. Pat. No. 5,270,736, to Inoue et al., entitled “Light modulation method,” describes a series of modulation methods for use in laser printers in which light pulses are provided trapezoidal pulse shapes (in time), such that pulses are ramped from below threshold to a peak current, are perhaps held at that level for some time, and then are ramped down again below laser threshold.
As other examples, commonly assigned U.S. Pat. No. 5,081,631 to Dhurjaty, entitled “Direct modulation of laser diodes for radiographic printers,” and commonly assigned U.S. Pat. No. 5,764,664, to Yip et al., entitled “Direct modulation method for laser diode in a laser film printer,” also describe approaches for direct laser modulation applicable to laser printing. Generally, these laser modulation circuits can also include corrective techniques, to compensate for laser behavior changes by changing the pulse modulation parameters, on at least a line-by-line basis. However, while such art provides foundational material for direct laser modulation, in these systems, individual lasers are writing the image data directly, rather than being used in aggregate as illumination sources to spatial light modulator array devices. As a result, this art does not anticipate the issues that arise when using multiple pulsed laser sources in illumination or in light valves or panels.
It is also recognized that electronic imaging systems have been developed in which one or more pulsed light sources are used to illuminate a modulator array device. As one example, U.S. Pat. No. 6,008,929 to Akimoto et al., entitled “Image displaying apparatus and method,” describes a projector in which a modulated light source illuminates a spatial light modulator array (a ferroelectric LCD), that is then imaged to the screen. In this system, the modulator array is fast, but provides binary (only “on” or “off” states) rather than grey scale operation. To compensate, a pulsed light sources is used, which is faster yet relative to rise or fall times, and which can be-periodically modulated with a variable duty cycle. Thus the pulsed light source provides the short, intermediate, and long light pulses that can be used to provide the required bit depth (i.e., tone levels). On a pixel-wise basis, the modulator array determines which pulses are used for the image content associated with a given frame time.
As another example, commonly assigned U.S. Pat. No. 6,621,615 to B. Kruschwitz et al., entitled “Method and system for image display,” describes a projection display in which a pulsed laser source is used to illuminate a spatial light modulator array, which image modulates the light that is then projected to the screen. The image modulation signals applied to the pixels of the SLM array are provided as pulse width modulation by a controller. The gray level of a pixel is controlled by switching the pixel into the “on” state for a controlled time, which is a multiple of a least-significant-bit (LSB) time. This system can use pulse number modulation in which the periodically-pulsed laser provides illuminating pulses of light and the pixel-wise modulation of the SLM determines the number of laser pulses that are passed through to the screen. However, Kruschwitz et al. observe that in the case that the laser pulse repetition rate is longer than the LSB time, then LSBs can be lost, and images are not faithfully reproduced. To counter this, the modulation controller of Kruschwitz et al. can apply a variable width switching profile to each pixel, such that the rise times and/or fall times for the LSBs are extended to exceed the pulse repetition rate.
Imaging devices have also been provided in which arrays of light sources are combined to illuminate a modulator array. For example, commonly assigned U.S. Pat. No. 6,215,547 to S. Ramanujan et al., entitled“Reflective liquid crystal modulator based printing system,” describes a printer using a reflective liquid crystal modulator and LED array illumination. In this instance, the light source comprises a two-dimensional array of LEDs at three distinct wavelengths representing red, blue, and green emission. The LEDs can be arranged in a mixed color array, with differing numbers of RGB LEDs in the array, the quantity being related to the media sensitivity. Light from the LEDs is collected, homogenized, and directed onto a SLM (a polarization switching LCD). The LEDs are then activated in a color sequential manner. Grayscale printing levels are provided using PWM of an LCD and the LEDs of a color channel in combination. The LEDs of color channel are activated as a group, and the activation cycle consists of a series of pulses, which may vary in duration or amplitude. The length and duration of the pulses are determined by the level of illumination needed per image to define the gray scale and by the sensitivity of media to light level and illumination time.
U.S. Patent Application Publication 2008/0185978, to J. Jeong et al., entitled “Light source driving apparatus, display device having the same, and driving method thereof,” describes a laser based projection display in which a plurality of laser devices is used to illuminate a spatial light modulator (a DMD). The multiple laser devices are partitioned into blocks, with one or more lasers devices per block, and they are then driven in a time division or phase offset manner by a switch element. A laser driver provides input to the blocks of laser devices, driving them at a high frequency (200 kHz) and low duty cycle (20%) to provide nominally equivalent output pulse intensities. As a result, the instantaneous current loads on the laser driver are time averaged, and a continuous pulse of light, synthesizing a CW illumination, can be provided to the DMD device for pixel specific PWM based on the image content at a given point in time. Jeong et al. make the assumption that the laser light sources are driven according to optimal frequencies, duty cycles, drive currents, and optical intensities in accordance with the specifications for the laser devices, and that the optical intensities provided by the various lasers devices are equal in output. There are no provisions for optical or electrical monitoring or feedback control of individual laser devices or groups of lasers to correct for changes in laser device behavior. Additionally, Jeong et al. do not consider the interaction of the laser pulses with the PWM of DMD array with respect to providing faithful reproductions of the image content. Jeong et al. also do not consider the interaction of operating multiple color channels simultaneously, each comprising an array of pulsed laser sources, to provide color image content, including the maintenance of color image quality as the performance of laser devices in the various color channels changes with time.
Thus, it is seen that the prior art, whether considered individually or in combination, does not anticipate or teach a method for providing and maintaining image quality, and particularly color image quality, in a laser based projection display that uses a plurality of PWM lasers, operable within a limited parameter range, for illumination in at least one color channel of the laser projector.